simmer

v.

1650s, alteration of simperen "to simmer" (late 15c.), possibly imitative; not thought to be connected to simper (v.). OED says the change is "probably due to a feeling of phonetic appropriateness." Figurative sense, of feelings, "to be agitated" is from 1764. Opposite sense, in simmer down, first recorded 1871, probably from the notion of moving from a full boil to a mere simmer.

I must and will keep shady and quiet till Bret Harte simmers down a little. [Mark Twain, letter, 1871]

Related: Simmered; simmering. The noun meaning "a condition of simmering" is from 1809.

simmer down

Become calm after anger or excitement, as in Simmer down, Mary; I'm sure he'll make it up to you, or I haven't time to look at your report now, but I will when things have simmered down a bit. This idiom derives from simmer in the sense of “cook at low heat, below the boiling point.” [Second half of 1800s]